1. Field of the Invention
The present invention relates to a system and method for downconverting the frequency of a communication signal, and more particularly to a system for frequency dehopping spread spectrum communication signals, the system including a receiver which includes a downconverter, with two mixer stages, each mixer stage providing coarse frequency dehopping of the input communication signal, wherein the local oscillator injection of each mixer stage is selectable from a plurality of tones, the output of the two mixer stages being sampled by an analog to digital converter to enable fine dehopping of the communication signal to be performed digitally.
2. Description of the Prior Art
Spread spectrum communication systems are known in the art. Such communication systems are normally used for secured communication in both military and satellite applications. Examples of such spread spectrum communication system are disclosed in U.S. Pat. Nos. 4,267,592; 4,630,282; 4,761,796; 4,914,699; 5,126,682; and 5,440,636.
Various spread spectrum communication techniques are known, such as direct sequence and frequency hopping. In direct sequence spread systems, a pseudo-noise signal, such as Gold code is employed. In particular, the pseudo-noise signal consists of a pseudo-random binary signal that is multiplied by the original base band bitstream to produce a new bitstream. In frequency hopping systems, the carrier or center frequency of the signal simply jumps to a number of different frequencies in a agreed upon pseudo-random binary sequence.
In both techniques, only those receivers which have the correct pseudo-random binary sequence can decode the original information. In order for a receiver to decode the information in a spread spectrum communication signal, the time sequential frequency pattern of the receiver is synchronized with the transmitter. In particular, once the receiver is synchronized with the transmitter, the spread spectrum signal is despread and subsequently demodulated to extract the original information from the spread spectrum signal. Despreading, also known as frequency dehopping, is usually done in the receiver by injecting a local oscillator signal to convert the spread spectrum signal into a signal having a frequency bandwidth sufficient to extract the information. Once the spread spectrum signal is despread or frequency dehopped, the desired information is extracted by well known demodulation techniques.
As mentioned above, spread spectrum communications systems are known to be used in satellites. In many such satellite systems, the weight allocation of the equipment in the satellite is a design constraint. In known receivers used in such satellites systems, frequency dehopping is accomplished in a single mixer stage with the local oscillator injection coming from one of a set of hopping frequency synthesizer units. The local oscillator signals from such synthesizers units are known to be routed through a relatively heavy and complex matrix switch in order to achieve key (or hopping pattern) to beam connectivity. Since the local oscillator signals in such a system can be in the 13 GHz region, the line losses in the coaxial cables and the switch matrix must be compensated by providing a high power output from each synthesizer unit. Moreover, the synthesizers used in such systems are required to have relatively high spectral purity requirements which increase the complexity and weight of such synthesizers.